Gaseous lasers have found extensive applications in the laser processing industry, including laser cutting, welding of materials, laser hardening through phase transformation, and in medical applications. In particular, in recent years, there has been considerable investigation into various forms of carbon dioxide gas (CO2) lasers, which radiate at wavelengths between 9 and 11 μm, and may be operated in Continuous Wave (CW) or pulsed regimes. While other gas lasers have efficiency of 0.1% or less, the CO2 laser may have efficiency of up to about 30%.
For excitation of the CO2 lasers, it is known to utilize DC (direct current) pulsed electric discharge and/or RF (radio frequency) alternating electric discharge. It has been suggested in the prior art to utilize combined DC and RF discharges for the excitation of CO2 cylindrical lasers. The prior art cylindrical lasers utilizing a combined DC and RF discharge operate with optically stable resonators that normally comprise, disposed at opposite ends of a laser cavity, a highly reflective output mirror which functions both to reflect internal radiation beams into the laser cavity and to transfer an output radiation beam exiting out of the laser cavity, and a feedback mirror. The two mirrors allow the internal beams to numerously oscillate inside the laser cavity in order to get high gain and improved directionality of the output beam.
A problem exists with the use of optically stable resonators in lasers where high output power is achieved by the increase of the laser tube inner diameter. Namely, it is known that an optically stable resonator operates in a multi-mode regime and produces a low quality laser beam, when its Fresnel number NF=α2/(λL) exceeds the value of 2, where α is a radial dimension of an exposed output mirror surface, λ is the wavelength of radiation inside the resonator, and L is the resonator length.
It has been known in the prior art to provide a powerful laser equipped with an unstable optical resonator with a relatively large Fresnel number (NF>3). The unstable resonator has primary and feedback mirrors, wherein the primary mirror is of a larger diameter than the feedback mirror so that the output radiation reflected from the periphery of the primary mirror is directed out of the laser cavity in a ring shaped beam surrounding the feedback mirror. The unstable resonator produces high optical quality beam, which may extract energy out of the entire gain volume. Furthermore, in view of the fact that the number of times the laser beam passes the laser cavity is small, the use of optically unstable resonators requires a specific care to be taken of the gain in the laser medium.
It is generally known that, in prior art gas lasers having an unstable resonator, the gain may be enhanced by the increase of the gas pressure. However, usage of high pressure in pulsed lasers normally decreases the pulse repetition frequency since the gas needs a relatively long time in order to recover. Hence, conventional pulsed lasers having an unstable resonator cannot operate with high pressure and, therefore, pulses provided thereby are normally of relatively small averaged power, which inevitably limits their applications.
It is also generally known that a hybrid unstable resonator is applied in laser cavities with large Fresnel number. Such resonators are generally used with asymmetrical gain medium lasers such as CO2 RF excited slab lasers. For the same reasons indicated above regarding unstable resonators, it is not efficient to increase the power of lasers which utilize hybrid resonators by increasing the gas pressure. Therefore, the main advantage of the hybrid unstable resonator is to operate in relatively low gain lasers with high efficiency.
The hybrid unstable resonator yields a laser beam which is diffraction limited on one axis and with multi mode degraded quality on the other axis. However, the effective beam quality is better than then stable resonator laser beam quality.